Peak flow in a peri-urban watershed: Past, present and future land use patterns

Background/Question/Methods

The Viviry watershed, located at the western edge of the Montreal urban area, has a mix of agricultural, forest, residential and recreational land cover. In the last 80 years this peri-urban region has seen extensive change. The historical landscape of agriculture, forest and sparse human settlement has become a forested residential area - a change in land use that has increased flows in the Viviry River. Compared to forests and farmland, developed areas with more impermeable surfaces increase runoff for a given precipitation event. Hence, forests, wetlands and agricultural fields provide a significant ecosystem service in this peri-urban landscape: flood mitigation. There is a direct tradeoff between development within the watershed and increased flood risk. The purpose of this study is to quantify the difference in peak flows between historical, present and future land use patterns, therefore quantifying the ecosystem service provided by natural spaces within the watershed. The scenarios analyzed are 1) historical land use (1933), 2) present day land use, 3) approved future development plans and 4) densification of development, with certain green spaces set aside for conservation under proposed local development plans.

Results/Conclusions

We calculated peak flows under the four land use scenarios and found that they have increased since 1933, and will continue to increase in the future if current development plans are followed. Historical peak flows are 27% lower than present day. This change can largely be attributed to development at the expense of forests and agricultural fields. While approved future development plans (scenario 3) will not significantly increase peak flows (1% increase), longer term development plans for the region of Montreal (scenario 4) leave forests vulnerable to development and result in increased peak flows (22%). Without thoughtful planning, the ecosystem service of flood mitigation provided by the undeveloped areas within the watershed will be reduced considerably. This analysis demonstrates the need to consider ecosystem services and watershed boundaries when planning for the conservation of green spaces.